Support multiple content types per path

[keorn]

When attempting to specify two routes with the same path and method, but different accepted format (e.g. format = "application/json" vs format = "application/cbor") error message appears: An OpenAPI operation has already been added for ...

The reason is that currently only method and path are taken into account when adding a new routes, while format should be considered as well.

(Thanks a lot for a nice crate!)